Source-collector module wth GIC mirror and LPP EUV light source

ABSTRACT

A source-collector module for an extreme ultraviolet (EUV) lithography system, the module including a laser-produced plasma (LPP) that generates EUV radiation and a grazing-incidence collector (GIC) mirror arranged relative thereto and having an input end and an output end. The LPP is formed using an LPP target system wherein a pulsed laser beam travels on-axis through the GIC and is incident upon solid, moveable LPP target. The GIC mirror is arranged relative to the LPP to receive the EUV radiation therefrom at its input end and focus the received EUV radiation at an intermediate focus adjacent the output end. An example GIC mirror design is presented that includes a polynomial surface-figure correction to compensate for GIC shell thickness effects, thereby improve far-field imaging performance.

CLAIM OF PRIORITY

This application claims the benefit of priority under 35 U.S.C. §119(e)of U.S. Provisional Application Ser. No. 61/335,700 filed on Jan. 11,2010, which application is incorporated by reference herein.

FIELD

The present disclosure relates generally to grazing-incidence collectors(GICs), and in particular to a source-collector module for use in anextreme ultraviolet (EUV) lithography system that employs alaser-produced plasma.

BACKGROUND ART

Laser-produced plasmas (LPPs) are formed in one example by irradiatingSn droplets with a focused laser beam. Because LPPs can radiate in theextreme ultraviolet (EUV) range of the electromagnetic spectrum, theyare considered to be a promising EUV radiation source for EUVlithography systems.

FIG. 1 is a schematic diagram of a generalized configuration for a priorart LPP-based source-collector module (“SOCOMO”) 10 that uses anormal-incidence collector (“NIC”) mirror MN, while FIG. 2 is a morespecific prior art example configuration of the “LPP-NIC” SOCOMO 10 ofFIG. 1. The LPP-NIC SOCOMO 10 includes a high-power laser source 12 thatgenerates a high-power, high-repetition-rate laser beam 13 having afocus F13. LPP-NIC SOCOMO 10 also includes along an optical axis A1 afold mirror FM and a large (e.g., ˜600 mm diameter) ellipsoidal NICmirror MN (having an on axis aperture hole to allow passage of the laserbeam 13 to the target) that includes a surface 16 with a multilayercoating 18. The multilayer coating 18 is essential to guarantee goodnear normal mirror reflectivity at EUV wavelengths. LPP-NIC SOCOMO 10also includes a Sn pellet (droplet) source 20 that emits a stream of Snpellets (droplets) 22 that pass through focus F13 for the laser beam 13.

In the operation of LPP-NIC SOCOMO 10, laser beam 13 irradiates Snpellets (droplets) 22 as the pellets pass through the focus F13 for thelaser beam 13, thereby produce a high-power LPP 24. LPP 24 typicallyresides on the order of hundreds of millimeters from NIC mirror MN andemits EUV radiation 30 as well as energetic Sn ions, particles, neutralatoms, and infrared (IR) radiation. The portion of the EUV radiation 30directed toward NIC mirror MN is collected by the NIC mirror MN and isdirected (focused) to an intermediate focus IF to form an intermediatefocal spot FS.

Advantages of LPP-NIC SOCOMO 10 are that the optical design is simple(i.e., uses a single ellipsoidal NIC mirror) and the nominal collectionefficiency can be high because NIC mirror MN can be designed to collecta large angular fraction of the EUV radiation 30 emitted from LPP 24. Itis noteworthy that the use of the single-bounce reflective NIC mirror MNplaced on the opposite side of LPP 24 from the intermediate focus IF,while geometrically convenient, requires that the Sn pellet (droplet)source 20 not significantly obstruct EUV radiation 30 being deliveredfrom the NIC mirror MN to the intermediate focus IF. Thus, there isgenerally no obscuration in the LPP-NIC SOCOMO 10 except perhaps for thehardware needed to generate the stream of Sn pellets (droplets) 22.

LPP-NIC SOCOMO 10 works well in laboratory and experimental arrangementswhere the lifetime and replacement cost of LPP-NIC SOCOMO 10 are notmajor considerations. However, a commercially viable EUV lithographysystem requires a SOCOMO that has a long lifetime. Unfortunately, theproximity of the surface 16 of NIC mirror MN and the multilayer coatings18 thereon to LPP 24, combined with the substantially normally incidentnature of the radiation collection process, makes it highly unlikelythat the multilayer coating 18 will remain undamaged for any reasonablelength of time under typical EUV-based semiconductor manufacturingconditions. The damage can come from ions incident on the multilayercoating 18 causing mixing and or absorption of EUV radiation 30; from Snatoms which could coat the multilayer coating 18 and thereby inhibitreflection of the EUV radiation 30; from thermal loading; and/or fromionizing EM radiation; and/or from energetic electrons.

A further drawback of the LPP-NIC SOCOMO 10 is that it cannot easily beused in conjunction with a physical debris mitigation device (DMD)because the DMD would obstruct the EUV radiation 30 from being reflectedfrom NIC mirror MN. In addition the NIC architecture using a highrep-rate droplet target places precise rep-rate demands on the lasersystem which adds to the cost of the laser system and adds additionalreliability risk to the SOCOMO system.

Multilayer coating 18 is also likely to have its performancesignificantly reduced by the build-up of Sn. Even a few nanometers ofsuch build-up will significantly absorb the EUV radiation 30 and reducethe reflectivity of the multilayer coating 18. Also, the aforementionedenergetic ions, atoms and particles produced by LPP 24 will bombardmultilayer coating 18 and can destroy the layered order of the toplayers of the multilayer coating 18. In addition, the energetic ions,atoms and particles will erode multilayer coating 18, and the attendantthermal heating from the generated IR radiation can act to mix orinterdiffuse the separate layers of the multilayer coating 18.

While a variety of subsystems have been proposed to mitigate theabove-identified problems with LPP-NIC SOCOMO 10, they all addsubstantial cost, reliability risk and complexity to the SOCOM system,to the point where it becomes increasingly unrealistic to include it ina commercially viable EUV lithography system. What is needed thereforeis a less expensive, less complex, more robust and generally morecommercially viable SOCOMO for use in an EUV lithography system thatuses an LPP-based EUV radiation source.

SUMMARY

The present disclosure is generally directed to grazing incidencecollectors (GICs), and in particular to GIC mirrors used to form asource-collector module (SOCOMO) for use in EUV lithography systems thathave a LPP-based EUV light source based on a movable Sn target with amuch simpler architecture then a droplet system—such as describedearlier. Such simple architecture could include a rotating Sn-coatedwheel or disk. Aspects of the LPP-GIC SOCOMO include a debris-mitigationdevice arranged between the LPP source and GIC mirror to reduce thethermal and debris load on the GIC and thereby extend the lifetime ofthe GIC mirror.

An aspect of the invention is a source-collector module for an extremeultraviolet (EUV) lithography system. The source-collector moduleincludes a laser, a solid laser-produced plasma (LPP) target and agrazing-incidence collector (GIC) mirror. The laser generates a pulsedlaser beam along a source-collector module axis. The solidlaser-produced plasma (LPP) target has a surface configured to receivethe pulsed laser beam and create an LPP that generates EUV radiation.The grazing-incidence collector (GIC) mirror has an input end and anoutput end. The GIC mirror is arranged to receive the EUV radiation atthe input end and focus the received EUV radiation at an intermediatefocus adjacent the output end.

In the source-collector module, the GIC mirror preferably provides afirst reflecting surface that does not have a multilayer coatingcovering a significant portion of the first reflecting surface.

In the source-collector module, the GIC mirror preferably includes a Rucoating.

In the source-collector module, the GIC mirror preferably includes amultilayer coating.

In the source-collector module, the GIC mirror preferably includes atleast one segmented GIC shell having the first reflecting surface and asecond reflecting surface, with the second reflecting surface having amultilayer coating.

In the source-collector module, the laser beam preferably travelsthrough the GIC from the output end to the input end and then to the LPPtarget surface.

In the source-collector module, the LPP target surface preferablyincludes a coating of LPP-generating material formed atop a substrate.

In the source-collector module, the LPP target surface preferablyincludes one of Sn and Xe.

The source-collector module preferably further includes aradiation-enhancement collection device disposed between the GIC mirroroutput end and the intermediate focus and configured to direct EUVradiation to the focus spot.

Another aspect of the invention is an extreme ultraviolet (EUV)lithography system for illuminating a reflective reticle. The EUVlithography system includes the above-mentioned source-collector moduleand an illuminator. The illuminator is configured to receive the focusedEUV radiation formed at the intermediate focus and from condensed EUVradiation for illuminating the reflective reticle.

The EUV lithography system preferably forms a patterned image on aphotosensitive semiconductor wafer. The EUV lithography systempreferably further includes a projection optical system. The projectionoptical system is arranged downstream of the reflective reticle andconfigured to receive reflected EUV radiation from the reflectivereticle and form therefrom the patterned image on the photosensitivesemiconductor wafer.

Another aspect of the invention is a method of collecting extremeultraviolet (EUV) radiation from a laser-produced plasma (LPP). Themethod includes providing a grazing incidence collector (GIC) mirroralong an axis. The GIC mirror having input and output ends. The methodalso includes arranging adjacent the GIC mirror input end an LPP targetsystem having a solid LPP target with a target surface. The method alsoincludes sending a pulsed laser beam down the GIC axis and through theGIC from the output end to the input end and to the target surface whilemoving the LPP target to form the LPP that emits the EUV radiation. Themethod further includes collecting with the GIC mirror at the GIC inputend a portion of the EUV radiation from the LPP and directing thecollected radiation out of the GIC mirror output end to form a focusspot at an intermediate focus.

The method preferably further includes employing a radiation collectionenhancement device arranged between the GIC output end and theintermediate focus to direct EUV radiation to the focus spot that wouldnot otherwise be directed to the focus spot by the GIC mirror.

The method preferably further includes providing the target as a movablesubstrate having a coating of LPP-generating material formed thereon.The LPP-generating material includes Sn or Xe.

In the method, the substrate preferably has a surface and an edge. Andthe method preferably further includes providing the coating on at leastone of the surface and the edge.

The method preferably further includes providing the GIC mirror with afirst reflecting surface that does not have a multilayer coating.

In the method, the GIC mirror preferably includes a Ru coating.

In the method, the GIC mirror preferably includes a multilayer coating.

In the method, the GIC mirror preferably includes at least one segmentedGIC shell that includes the first reflecting surface and a secondreflecting surface, with the second reflecting surface having themultilayer coating.

The method preferably further includes forming from EUV radiation at theintermediate focus condensed EUV radiation for illuminating a reflectivereticle.

The method preferably further includes receiving reflected EUV radiationfrom the reflective reticle and forming therefrom the patterned image onthe photosensitive semiconductor wafer using a projection opticalsystem.

Another aspect of the invention is a grazing incidence collector (GIC)mirror having an input end and an output end and for use with alaser-produced plasma (LPP) target system that generates an LPP thatemits extreme ultraviolet (EUV) radiation. The GIC mirror includes threeconcentrically arranged innermost GIC shells and five outermost GICshells. The three concentrically arranged innermost GIC shells have anelliptical shape. The five outermost GIC shells concentrically surroundthe three innermost GIC shells. And the five outermost GIC shellsprovide a double reflection of the EUV radiation, with each of theoutermost GIC shells having a curvature defined by revolving anelliptical section and a hyperbolic section around a common axis notcoincident with respective ellipse and hyperbola axes. And the fiveoutermost GIC shells are respectively configured to provide a singlereflection for EUV radiation that enters the input end and exits theoutput end.

In the GIC mirror, each GIC shell has a thickness. The GIC shellpreferably includes a polynomial surface-figure correction to uniformizevariations in an intermediate image due to the GIC shell thickness.

Another aspect of the invention is a source collector module (SOCOMO)for an extreme ultraviolet (EUV) lithography system. The SOCOMO includesthe above-mentioned GIC mirror and the LPP target system. The LPP targetsystem is configured so that the LPP is formed adjacent the input end ofthe GIC mirror.

Additional features and advantages of the disclosure are set forth inthe detailed description below, and in part will be readily apparent tothose skilled in the art from that description or recognized bypracticing the disclosure as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a generalized example prior art LPP-NICSOCOMO;

FIG. 2 is a schematic diagram of a particular example of a prior artLPP-NIC SOCOMO in accordance with FIG. 1;

FIG. 3A is a generalized schematic diagram of an example GIC-basedSOCOMO for an LPP source (“LPP-GIC SOCOMO”), wherein the LPP andintermediate focus are on opposite sides of the GIC mirror;

FIG. 3B is similar to FIG. 3A, but illustrating an example LPP-GICSOCOMO that includes a radiation-collection enhancement device (RCED);

FIG. 4, FIG. 5A, FIG. 5B, and FIGS. 6A and 6B are schematic diagrams ofexample LPP-GIC SOCOMOs according to the present disclosure and thegeneralized configuration of FIG. 3;

FIG. 7A is a detailed schematic side view of the LPP target system ofFIG. 6A;

FIG. 7B is a cross-sectional view of an example LPP target made up of asubstrate and a coating of an LPP-generating material formed thereon;

FIG. 7C is a schematic example of a front-on view of the LPP targetsystem of FIG. 7A, showing how the target handler moves new LPP targetsfrom the storage cassette to the mounting plate.

FIG. 7D is similar to FIG. 7A and shows an example LPP target systemthat utilizes two target assemblies, wherein one target assembly is “online” and generating the LPP while the other target assembly is “offline” for LPP target removal and replacement;

FIG. 8A is a detailed schematic side view of the example LPP targetsystem shown in FIG. 6B and that includes a disc-type LPP target withthe target surface on the edge of the disc rather than on the frontsurface;

FIG. 8B is a perspective view and FIG. 8C is a cross-sectional view ofan example LPP target of FIG. 8A, as taken along the line 8C-8C, showingan example target substrate with its edge coated with an LPP-generatingmaterial;

FIG. 8D is a close-up view of example embodiment of a portion of thetarget assembly illustrating a cooled configuration for the targetassembly;

FIG. 9 is a cross-sectional diagram of an example GIC mirror having twosections with respective first and second surfaces that provide firstand second reflections of EUV radiation;

FIG. 10 is a schematic diagram similar to FIG. 3 and that illustrates anembodiment of a LPP-GIC SOCOMO with a debris mitigation device arrangedbetween the LPP and GIC mirror and an optional RCED;

FIG. 11 is a schematic cross-sectional diagram of a portion of anexample GIC mirror showing two of the two-section GIC shells used in theouter portion of the GIC mirror;

FIG. 12 is a schematic cross-sectional diagram of a portion of the GICmirror of FIG. 11 showing all eight GIC shells and the LPP;

FIG. 13A is a plot of the normalized far-field position vs. Intensity(arbitrary units) for the case where the GIC shells do not include apolynomial surface-figure correction to improve the far-field imageuniformity;

FIG. 13B is the same plot as FIG. 13A, but with a polynomialsurface-figure correction that improves the far-field image uniformity;and

FIG. 14 is a schematic diagram of an EUV lithography system thatutilizes the LPP-GIC SOCOMO of the present disclosure.

The various elements depicted in the drawing are merely representationaland are not necessarily drawn to scale. Certain sections thereof may beexaggerated, while others may be minimized. The drawing is intended toillustrate an example embodiment of the disclosure that can beunderstood and appropriately carried out by those of ordinary skill inthe art.

DETAILED DESCRIPTION

The present disclosure is generally directed to GICs, and in particularto GIC mirrors used to form a source-collector module (SOCOMO) for usein EUV lithography systems that have a LPP-based EUV light source.Aspects of the LPP-GIC SOCOMO may include a debris-mitigation device(DMD) arranged between the LPP and GIC mirror to extend the lifetime ofthe GIC mirror.

FIG. 3A is a generalized schematic diagram of example LPP-GIC SOCOMO100, wherein LPP 24 and intermediate focus IF are on opposite sides of aGIC mirror MG. GIC mirror MG has an input end 3 and an output end 5. AnLPP target system 40 that generates LPP 24 is also shown, and examplesof the LPP target system 40 are discussed in detail below.

FIG. 3B is similar to FIG. 3A, but illustrating an example LPP-GICSOCOMO 100 that includes a radiation-collection enhancement device(RCED) 37 arranged adjacent an aperture stop AS at or near where anintermediate focus spot FS is formed. RCEDs 37 are discussed in U.S.Provisional Patent Application No. 61/341,806, entitled “EUV collectorsystem with enhanced EUV radiation collection,” which application isincorporated by reference herein. RCED 37 is disposed between the outputend of GIC mirror MG and the intermediate focus spot FS and isconfigured to direct EUV radiation 30 to the intermediate focus spot FSthat would not otherwise contribute to forming the intermediate focusspot FS.

FIG. 4 is a schematic diagram of an example LPP-GIC SOCOMO 100 based onthe general configuration of FIG. 3A. LPP-GIC SOCOMO 100 of FIG. 4utilizes an LPP target system 40 having a Sn pellet (droplet) source 20that generates the aforementioned series of Sn pellets (droplets) 22. Inthe present embodiment, Sn pellets (droplets) 22 are relatively low-masspellets that when irradiated with laser beam 13 generate substantiallyisotropic EUV radiation 30. This allows for a configuration wheremulti-shell GIC mirror MG (shown with two GIC shells M1 and M2) isarranged along optical axis A1 between LPP 24 and intermediate focus IF.A lens 17 assists in focusing laser beam 13 to focus F13. In an exampleembodiment, GIC shells M1 and M2 include Ru coatings, which arerelatively stable and can tolerate a certain amount of Sn coating fromLPP 24 without significantly reducing the reflectivity of the GICmirrors MG.

LPP-GIC SOCOMO 100 of FIG. 5A is similar to that of FIG. 4, except thatSn pellets (droplets) 22 are relatively high-mass pellets that result inan anisotropic emission of EUV radiation 30 when irradiated by focusedlaser beam 13. In LPP-GIC SOCOMO 100 of FIG. 5A, laser source 12,focusing lens 17 and fold mirror FM are arranged so that Sn pellets(droplets) 22 are irradiated in the −X direction along optical axis A1,thereby creating EUV radiation 30 that is emitted substantially in the+X direction. The axial obscuration presented by fold mirror FM isminimal because of the finite diameter of the innermost GIC shell. Thus,laser beam 13 travels in one direction through GIC mirror MG generallyalong optical axis A1 and EUV radiation 30 travels in the oppositedirection through the GIC mirror MG and to intermediate focus IF.

FIG. 5B is similar to FIG. 5A, but with the LPP-GIC SOCOMO 100 includingan RCED 37 arranged adjacent aperture stop AS at or near whereintermediate focus spot FS is formed.

LPP-GIC SOCOMO 100 of FIG. 6A is similar to that of FIG. 5A, except thatLPP target system 40 includes a relatively high-mass, solid, moveableLPP target 27 having a surface 29. In various examples, LPP targetmaterial may include Sn or Xe. LPP target system 40 includes a targetassembly 41 having target driver 31 (e.g., a motor), a drive shaft 32attached to the target driver 31, and a mounting plate MP attached tothe drive shaft 32. LPP target 27 mounts to mounting plate MP. Theincident laser beam 13 from laser source 12 is directed to travelaxially through GIC mirror MG in the −X direction and is incident uponthe surface 29 of LPP target 27 to form LPP 24. Moving LPP target 27allows for laser beam 13 to be incident upon surface 29 of LPP target 27at a different location for each laser pulse, or to have a limitednumber of laser pulses for each target location. The EUV radiation 30from LPP 24 formed on LPP target 27 is generally emitted in the +Xdirection and travels through GIC mirror MG in the opposite direction oflaser beam 13.

FIG. 6B is similar to FIG. 6A and illustrates an example embodiment of aLPP target system 40 having a solid LPP target 27 in the form of arotating disc, where the surface 29 of LPP target 27 is now the discedge rather than the disc face. This embodiment for LPP target system 40is discussed in greater detail below. Both embodiments of LPP-GIC SOCOMO100 of FIGS. 6A and 6B include an optional RCED 37.

Example Target Embodiments

FIG. 7A is a detailed schematic side view of LPP target system 40 ofFIG. 6A. LPP target system 40 includes a vacuum chamber 42 having aninterior 43 and an opening 44 surrounded by a flange 45 used to connectvacuum chamber 42 to a larger vacuum chamber (not shown) for the LPP-GICSOCOMO 100.

LPP target system 40 includes the aforementioned target assembly 41 thatsupports LPP target 27. Target assembly 41 is configured to translateand/or rotate LPP target 27 so that laser beam 13 is scanned (e.g.,raster scanned, spiral scanned, etc.) over surface 29 of LPP target 27.

With reference to FIG. 7B, in one embodiment, LPP target 27 is asubstrate 50 (e.g., glass, ceramic, metal, etc.) having a coating 52made of an LPP-generating material such as Sn or Xe. An examplethickness TH of coating 52 is between 1 micron and 5 microns. In oneembodiment, substrate 50 is a standard blank reticle used insemiconductor manufacturing, with the advantage that handling equipmentfor such substrates has been developed and is readily available. Inanother embodiment, substrate 50 is a standard blank compact-disc (CD)used for data storage, except that it is covered with coating 52.Standard handling equipment is also available for such substrates. Bothtypes of substrates can be inexpensively refurbished and reused.

With reference also to the front-on view of FIG. 7C, LPP target system40 also includes a first cassette 60N for storing new LPP targets 27 anda second cassette 60U for storing used LPP targets 27. LPP target system40 also includes target handling system 62 configured with a movabletarget holder 64 to remove used LPP targets 27 from mounting plate MPand move them (via path P1) to the second cassette 60U for storage andto remove (via path P2) new targets from first cassette 60N to themounting plate MP. In an example embodiment, target handling system 62makes use of vacuum interlocked robots. Other embodiments include moving(translating) target assembly 41 to move used LPP targets 27 to secondcassette 60U and pick up a new LPP target 27 from first cassette 60N.This particular embodiment is best suited for lightweight LPP targets27.

In another embodiment illustrated in FIG. 7D, a pair of targetassemblies 41, with one target assembly 41 being “off-line” to remove aused LPP target 27 and replace it with a new one, while the other targetassembly 41 is “on-line” and being used to generate LPP 24.

FIG. 8A is a detailed schematic side view of the example LPP targetsystem 40 shown in FIG. 6B that includes a disc-type LPP target 27, withthe surface 29 of LPP target 27 being on the edge of the disc ratherthan on its front surface. Target assembly 41 is still used to drive LPPtarget 27, but now the LPP target 27 is oriented so that laser beam 13strikes the edge of LPP target 27 (see laser spots 13S formed at angleφ) as the LPP target 27 is driven to rotate about the Z-axis.

FIG. 8B is a perspective view and FIG. 8C is a cross-sectional view ofan example LPP target 27 taken along the line 8C-8C that shows substrate50 with its edge 51 coated with coating 52 made from LPP-generatingmaterial. This configuration forms a coating 52 that defines surface 29of LPP target 27. An example coating width W is between 1 mm and 5 mm,and an example thickness TH of coating 52 is 1 micron to 5 microns.

The LPP target system 40 of FIG. 8A also includes a melting source 70that emits an energy beam 72 that irradiates surface 29 of LPP target 27on the side opposite where laser beam 13 is incident upon the surface 29of LPP target 27. Energy beam 72 serves to locally melt coating 52 torefurbish surface 29 of LPP target 27. Example melting sources 70include e-beam systems, lasers, heating elements, filaments, etc. Asurface finish monitor 80 is arranged relative to where energy beam 72is incident upon surface 29 of LPP target 27 to monitor the refurbished(refinished) surface 29 of LPP target 27. A fold mirror 81 is shown tofacilitate viewing the refurbished surface 29 of LPP target 27 bysurface finish monitor 80.

In an example embodiment, LPP target system 40 includes a controller 90operably connected to target driver 31, melting source 70 and surfacefinish monitor 80 and is configured to control the overall operation ofthe LPP target system 40. An example controller 90 includes a computerthat can store instructions (software) in a computer readable medium(memory) to cause the computer (via a processor therein) to carry outthe instructions to operate LPP target system 40 to generate LPP 24. Oneexample operation of controller 90 is to shift LPP target 27 in thedirection of laser beam 13 as the thickness TH of coating 52 decreaseswhile generating LPP 24.

FIG. 8D is a close-up view of example embodiment of a portion of targetassembly 41, wherein drive shaft 32 includes a cooling channel 33 with adivider 33D, and wherein LPP target 27 includes an internal targetchamber 28. Cooling channel 33 of drive shaft 32 and internal targetchamber 28 are fluidly connected so that a cooling fluid CF can flowfrom a cooling fluid source CFS through cooling channel 33 and to theinternal target chamber 28 and then back to the cooling fluid sourceCFS, as illustrated. Other cooling configurations are contemplatedbeyond this illustrative example. Such a cooling system is desirable forhigh-repetition-rates since the attendant differential heating ofcoating 52 can degrade surface 29 of LPP target 27.

SOCOMO with No First-Mirror Multilayer

An example configuration of LPP-GIC SOCOMO 100 has no multilayer-coated“first mirror,” i.e., the mirror or mirror section upon which EUVradiation 30 is first incident (i.e., first reflected) does not have amultilayer coating 18 that covers a significant portion of thereflecting surface. In another example configuration of LPP-GIC SOCOMO100, the first mirror is substantially a grazing incidence mirror. In anexample, only a small section (i.e., a non-significant portion) of thereflective surface has a multilayer coating 18, e.g., for diagnosticpurposes.

A major advantage of LPP-GIC SOCOMO 100 is that its performance is notdependent upon on the survival of a multilayer coated reflectivesurface. Example embodiments of GIC mirror MG have at least onesegmented GIC shell, such as GIC shell M1 shown in FIG. 9. GIC shell M1is shown as having a two mirror segments M1A and M1B with respectivefirst and second surfaces S1 and S2. First surface S1 provides the firstreflection (and is thus the “first mirror”) and second surface S2provides a second reflection that is not in the line of sight to LPP 24.In an example embodiment, second surface S2 supports a multilayercoating 18 since the intensity of the once-reflected EUV radiation 30 issubstantially diminished and is not normally in the line of sight of LPP24, thus minimizing the amount of ions and neutral atoms incident uponthe multilayer coating 18. Finally, radiation is also grazingly incidentupon the second surface S2 on the second reflection, thereby presentingless risk of damaging multilayer coating 18.

SOCOMO Lifetime

Another advantage of LPP-GIC SOCOMO 100 of the present disclosure isthat its anticipated lifetime is in excess of 1 year, which is acommercially viable lifetime for a EUV lithography system used insemiconductor manufacturing. Another advantage is that it supportsembodiments wherein the LPP target system 40 need not be based ondispensed Sn pellets (droplets) 22 but rather employs a solid LPP target27 (see e.g., FIG. 6A and FIG. 6B).

GIC vs. NIC SOCOMOs

There are certain trade-offs associated with using a LPP-GIC SOCOMO 100versus a LPP-NIC SOCOMO 10. For example, for a given collection angle ofthe EUV radiation 30 from the LPP 24, the LPP-NIC SOCOMO 10 can bedesigned to be more compact than the LPP-GIC SOCOMO 100.

Also, the LPP-NIC SOCOMO 10 can in principle be designed to collect EUVradiation 30 emitted from the source at angles larger than 90° (withrespect to the optical axis A1), thus allowing larger collectionefficiency. However, in practice this advantage is not normally usedbecause it leads to excessive NIC diameters or excessive angles that theEUV radiation 30 forms with the optical axis A1 at intermediate focusIF.

Also, the far field intensity distribution generated by a LPP-GIC SOCOMO100 has additional obscurations due to the shadow of the thickness ofthe GIC shells and of the mechanical structure supporting the GICmirrors MG. However, the present disclosure discusses embodiments belowwhere the GIC surface includes a surface correction that mitigates theshadowing effect of the GIC shells thicknesses and improves theuniformity of the intermediate focus spot FS at the intermediate focusIF.

Further, the intermediate focus spot FS at intermediate focus IF will ingeneral be larger for a LPP-GIC SOCOMO 100 than for a LPP-NIC SOCOMO 10.This size difference is primarily associated with GIC mirror figureerrors, which are likely to decrease as the technology evolves.

On the whole, it is generally believed that the above-mentionedtrade-offs are far outweighed by the benefits of a longer operatinglifetime, reduced cost, simplicity, and reduced maintenance costs andissues associated with a LPP-GIC SOCOMO 100.

LPP-GIC SOCOMO with Debris Mitigation

FIG. 10 illustrates an example embodiment of generalized a LPP-GICSOCOMO 100 similar to FIG. 3B but that includes a debris mitigationdevice (DMD) 200 arranged between LPP 24 and GIC mirror MG. DMD 200 isshown in phantom to illustrate the fact that it typically passes EUVradiation 30 while blocking other damaging ions and particles (e.g.,such as energetic Sn ions) from LPP 24. DMD 200 may also be configuredto remove debris, such as Sn, formed in GIC mirror MG. Optional RCED 37is shown in FIG. 10.

Example DMDs 200 include those used in LPP-NIC SOCOMO technology, suchas magnetic-field-based DMDs or DMDs based on a plurality of radialmetal lamellas. Example DMDs are discussed in U.S. Pat. Nos. 7,230,258,7,423,275, 7,372,049, 7,355,190 and 7,193,229, 7,180,083 and 6,963,071,which patents are incorporated herein by reference, and also in thearticle by D. J. W. Klunder, et al., “Debris Mitigation and CleaningStrategies for Sn-Based Sources for EUV Lithography,” Proceedings ofSPIE, vol. 5751, pp. 943-951, which article is incorporated by referenceherein.

Example GIC Mirror for LPP-GIC SOCOMO

FIG. 11 is a schematic side view of a portion of an example GIC mirrorMG for use in LPP-GIC SOCOMO 100. The optical design of GIC mirror MG ofFIG. 11 actually consists of eight nested GIC shells 210 withcylindrical symmetry around the optical axis A1, as shown in FIG. 12. Tominimize the number of GIC shells 210, the first three innermost GICshells 210 are elliptical, whereas the five outermost GIC shells 210 arebased on an off-axis double-reflection design having elliptical andhyperbolic cross sections, such as described in European PatentApplication Publication No. EP1901126A1, entitled “A collector opticalsystem,” which application is incorporated by reference herein. FIG. 11shows two of the outermost GIC shells 210 having an elliptical section210E and a hyperboloidal section 210H. FIG. 11 also shows the sourcefocus SF, the virtual common focus VCF, and the intermediate focus IF,as well as the axes AE and AH for the elliptical and hyperboloidal GICshells 210E and 210H, respectively. The distance between virtual commonfocus VCF and intermediate focus IF is ΔL. The virtual common focus VCFis offset from the optical axis A1 by a distance Δr. The full opticalsurface is obtained by a revolution of the cross sections of theelliptical and hyperboloidal GIC shells 210E and 210H around the opticalaxis A1.

Example designs for the example GIC mirror MG are provided in Table 1and Table 2 below. The main optical parameters of the design are: a) adistance ΔL between LPP 24 and intermediate focus IF of 2400 mm; and b)a maximum collection angle at the LPP side of 70.7°. In an exampleembodiment, GIC shells 210 each include a Ru coating for improvedreflectivity at EUV wavelengths. The nominal collection efficiency ofthe GIC mirror MG for EUV radiation 30 of wavelength of 13.5 nm when theoptical surfaces of GIC shells 210 are coated with Ru is 37.6% withrespect to 2π steradians emission from LPP 24.

Since an LPP EUV source is much smaller than a discharge-produced plasma(DPP) EUV source (roughly by a factor of 10), the use of LPP 24 allowsfor better etendue matching between the GIC mirror output and theilluminator input. In particular, the collection angle at LPP 24 can beincreased to very large values with negligible or very limitedefficiency loss due to mismatch between the GIC mirror MG andilluminator etendue. In an example embodiment, the collection angleexceeds 70°.

The dimension of LPP 24 has a drawback in that the uniformity of theintensity distribution in the far field tend to be worse than for a DPPsource, for a given collector optical design. Indeed, since the LLP 24is smaller, the far-field shadows due to the thicknesses of GIC shells210 tend to be sharper for an LPP source than for a DPP source.

To compensate at least partially for this effect, a surface figure(i.e., optical profile) correction is added to each GIC shell 210 toimprove the uniformity of the intensity distribution in the far field(see, e.g., Publication No. WO2009-095219 A1, entitled “Improved grazingincidence collector optical systems for EUV and X-ray applications,”which publication is incorporated by reference herein). Thus, in anexample embodiment of GIC mirror MG, each GIC shell 210 has superimposedthereon a polynomial (parabolic) correction equal to zero at the twoedges of the GIC shells 210 and having a maximum value of 0.01 mm.

Table 1 and Table 2 set forth an example design for the GIC mirror MGshown in FIG. 10. The “mirror #” is the number of the particular GICshell 210 as numbered starting from the innermost GIC shell 210 to theoutermost GIC shell 210.

TABLE 1 Hyperbola Ellipse Mirror radii [mm] Radius of Radius of Ellipse-Conic curvature Conic curvature hyperbola Mirror # Constant [mm]Constant [mm] Maximum joint Minimum 1 — — −0.990478  11.481350 83.347856 —  65.369292 2 — — −0.979648  24.674461 122.379422 — 94.644337 3 — — −0.957302  52.367323 179.304368 — 137.387744 4−1.066792 29.401382 −0.963621  61.100890 202.496127 192.634298152.384167 5 −1.072492 34.268782 −0.949865  86.379783 228.263879216.839614 169.639161 6 −1.090556 46.865545 −0.941216 104.704248257.297034 243.541412 188.559378 7 −1.111163 61.694607 −0.926716134.626393 293.432077 276.198514 208.671768 8 −1.134540 81.393448−0.905453 180.891785 340.258110 317.294990 229.102808

TABLE 2 Position of virtual common focus VCF with respect tointermediate focus IF ΔL, parallel to optical Δr, transverse to Mirroraxis A1 optical axis A1 # [mm] [mm] 1 — — 2 — — 3 — — 4 3293.000000171.500000 5 3350.000000 237.000000 6 3445.000000 276.300000 73521.000000 335.250000 8 3616.000000 426.950000

FIG. 13A is a plot of the normalized far-field position at theintermediate focus IF vs. intensity (arbitrary units) for light raysincident thereon for the case where there is no correction of the GICshell profile. The plot is a measure of the uniformity of theintermediate image (i.e., “intermediate focus spot” FS) of LPP 24 asformed at the intermediate focus IF. LPP 24 is modeled as a sphere witha 0.2 mm diameter.

FIG. 13B is the same plot except with the above-described correctionadded to GIC shells 210. The comparison of the two plots of FIG. 13A andFIG. 13B shows substantially reduced oscillations in intensity in FIG.13B and thus a significant improvement in the far field uniformity theintermediate focus spot FS at the intermediate focus IF as a result ofthe corrected surface figures for the GIC shells 210.

EUV Lithography System with LPP-GIC SOCOMO

FIG. 14 is an example EUV lithography system (“lithography system”) 300according to the present disclosure. Example EUV lithography systems aredisclosed, for example, in U.S. Patent Applications No.US2004/0265712A1, US2005/0016679A1 and US2005/0155624A1, which are areincorporated herein by reference.

Lithography system 300 includes a system axis ASy and an EUV lightsource LS that includes an LPP target system 40, such as one of thosediscussed above, which generates LPP 24 that emits working EUV radiation30 at λ=13.5 nm.

Lithography system 300 includes an EUV GIC mirror MG such as thatdescribed above. In an example embodiment, EUV GIC mirror MG is cooledas described in U.S. patent application Ser. No. 12/592,735, which isincorporated by reference herein. EUV GIC mirror MG is arranged adjacentand downstream of EUV light source LS, with collector axis AC lyingalong system axis ASy. EUV GIC mirror MG collects working EUV radiation30 (i.e., light rays LR) from EUV light source LS located at sourcefocus SF and the collected radiation forms intermediate source image IS(i.e., intermediate focus spot FS) at intermediate focus IF. In anexample, LPP-GIC SOCOMO 100 comprises EUV light source LS and GIC mirrorMG. Optional RCED 37 is shown by way of example.

An illumination system 316 with an input end 317 and an output end 318is arranged along system axis ASy and adjacent and downstream of EUV GICmirror MG with the input end 317 adjacent the EUV GIC mirror MG.Illumination system 316 receives at input end 317 EUV radiation 30 fromintermediate source image IS and outputs at output end 318 asubstantially uniform EUV radiation beam 320 (i.e., condensed EUVradiation). Where lithography system 300 is a scanning type system, EUVradiation beam 320 is typically formed as a substantially uniform lineor ring field of EUV radiation 30 at a reflective reticle (mask) 336that scans over the reflective reticle 336.

A projection optical system 326 is arranged along (folded) system axisASy downstream of illumination system 316. Projection optical system 326has an input end 327 facing output end 318 of illumination system 316,and an opposite output end 328. A reflective reticle 336 is arrangedadjacent the input end 327 of projection optical system 326 and asemiconductor wafer 340 is arranged adjacent output end 328 ofprojection optical system 326. Reflective reticle 336 includes a pattern(not shown) to be transferred to semiconductor wafer 340, which includesa photosensitive coating (e.g., photoresist layer) 342. In operation,the uniformized EUV radiation beam 320 irradiates reflective reticle 336and reflects therefrom, and the pattern thereon is imaged onto surfaceof photosensitive coating 342 of semiconductor wafer 340 by projectionoptical system 326. In a lithography scanning system 300, the reticleimage scans over the surface of photosensitive coating 342 to form thepattern over the exposure field. Scanning is typically achieved bymoving reflective reticle 336 and semiconductor wafer 340 in synchrony.

Once the reticle pattern is imaged and recorded on semiconductor wafer340, the patterned semiconductor wafer 340 is then processed usingstandard photolithographic and semiconductor processing techniques toform integrated circuit (IC) chips.

Note that in general the components of lithography system 300 are shownlying along a common folded system axis ASy in FIG. 14 for the sake ofillustration. One skilled in the art will understand that there is oftenan offset between entrance and exit axes for the various components suchas for illumination system 316 and for projection optical system 326.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present disclosurewithout departing from the spirit and scope of the disclosure. Thus itis intended that the present disclosure cover the modifications andvariations of this disclosure provided they come within the scope of theappended claims and their equivalents.

1. A source-collector module for an extreme ultraviolet (EUV)lithography system, comprising: a laser that generates along asource-collector module axis a pulsed laser beam having a series ofpulses; a solid laser-produced plasma (LPP) target having a movingtarget surface configured to receive the pulsed laser beam with eachlaser pulse being incident upon the target surface at a differentlocation to create a LPP that generates EUV radiation and debris; and agrazing-incidence collector (GIC) mirror having an input end and anoutput end and arranged to receive the EUV radiation at the input endand focus the received EUV radiation at an intermediate focus adjacentthe output end; a debris mitigation device operably disposed between thesolid LPP target and the input end of the GIC mirror to substantiallyprevent the debris from reaching the GIC mirror; and aradiation-collection enhancement device disposed between the GIC mirroroutput end and the intermediate focus and configured to direct EUVradiation to the focus spot.
 2. The source-collector module of claim 1,wherein the debris mitigation device comprises a plurality of radiallamellas that rotate to intercept the debris as the debris travels fromthe LPP to the GIC mirror.
 3. The source-collector module of claim 1,wherein the GIC mirror provides a first reflecting surface that does nothave a multilayer coating covering a significant portion of the firstreflecting surface.
 4. The source-collector module of claim 1, whereinthe GIC mirror includes a Ru coating.
 5. The source-collector module ofclaim 1, wherein the GIC mirror includes a multilayer coating.
 6. Thesource-collector module of claim 1, wherein the GIC mirror includes atleast one segmented GIC shell having the first reflecting surface and asecond reflecting surface, with the second reflecting surface having amultilayer coating.
 7. The source-collector module of claim 1, whereinthe laser beam travels through the GIC from the output end to the inputend, through the debris mitigation device and then to the LPP targetsurface.
 8. The source-collector module of claim 1, wherein the LPPtarget surface includes a coating of LPP-generating material formed atopa substrate.
 9. The source-collector module of claim 1, wherein the LPPtarget surface includes one of Sn and Xe.
 10. An extreme ultraviolet(EUV) lithography system for illuminating a reflective reticle,comprising: the source-collector module of claim 1; an illuminatorconfigured to receive the focused EUV radiation formed at theintermediate focus and form condensed EUV radiation for illuminating thereflective reticle.
 11. The EUV lithography system of claim 10 forforming a patterned image on a photosensitive semiconductor wafer,further comprising: a projection optical system arranged downstream ofthe reflective reticle and configured to receive reflected EUV radiationfrom the reflective reticle and form therefrom the patterned image onthe photosensitive semiconductor wafer.